This invention relates to energy storage systems. More particularly, it relates to methods and apparatuses for storing ice or eutectic material for efficiently and inexpensively supplying stored energy during peak electrical demand times.
In recent years, electric utilities have been pricing electricity using a two-tier system, particularly for commercial customers. The two-tier system is divided into peak hours and off-peak hours. Off-peak hours occur when electrical demand is the lowest, and peak hours occur when electrical demand is the highest. Off-peak hours generally occur late at night and early in the morning, while peak hours generally occur late in the morning and in the afternoon. The price of electricity is much higher during the peak hours of the day than during off peak hours. The ratio between peak and off peak prices often varies from 2:1 to 4:1. Furthermore, demand charges are quite often not levied during off-peak hours.
For many businesses, such as office buildings and restaurants, electrical energy usage, particularly for air conditioning, is a major portion of its operating costs. Therefore, in connection with air conditioning, it is advantageous to utilize high electrical use devices such as compressors only during off-peak times and to utilize stored energy during the peak times.
Various proposals have been suggested for accomplishing the above, some of which are set forth in The Complete Guide To Energy Storage For Cooling Commercial Buildings, published by McCannon Consulting Group of San Diego, Calif., in July of 1985.
Currently, there are two basic types of cold storage systems available. One type utilizes a large tank for storing chilled water. Compressors are utilized to chill the water down to about 40.degree. F. during off-peak hours. The chilled water is then circulated into the space to be conditioned in the building by means of a pump during peak hours. One major disadvantage of the chilled water system is that it requires a very large water storage tank to accomplish a substantial amount of cooling in most commercial buildings.
Other types of cool storage systems currently available are ice storage systems. Generally, the two types of ice storage systems are a static or ice building system and a dynamic or an ice shucking system. In a static system, a storage tank is built around cooling coils, whereby ice is formed around the coils. The water remaining in the tank is circulated into the building during peak demand hours and the ice on the coils melts as heat exchange occurs from the outside of the ice formation towards the cooling coils. One disadvantage of this coil system is that as ice builds on the outside of the coils, the ice itself creates an insulating effect progressively causing the use of more energy when the next ice-forming cycle occurs.
In the ice-shucking system, ice is made in sheet or crushed form and delivered for storage into large bins similar to those used in the water chilled system. While ice systems have advantages over the water chilled systems, particularly in that a smaller storage volume is required, colder temperatures are needed, resulting in the use of larger compressors and larger condensing units and more energy is spent per ton of ice produced.
Other types of ice storage systems have been recently developed. One type utilizes a glycol solution circulating through the coils in the tank. This system freezes substantially all the water in the tank. Some of the disadvantages of the glycol system are the added heat exchanger required to cool the glycol by refrigeration and a lower evaporator temperature which results in greater energy required to produce a given amount of cooling.